1. Field of the Invention
The present invention relates to a method and an apparatus for crystal growth wherein it becomes possible to regulate to a certain degree the composition and the impurity concentration in a crystal layer grown epitaxially in a liquid phase.
2. Description of the Prior Art
In a crystal growth process in a liquid phase, in the case where the crystal substance to be grown contains a constituent element having a segregation coefficient greater than 1, the concentration of the constituent element in the grown crystal layer decreases in the direction of growth. It is evident that the greater the segregation coefficient of the constituent element, the more prominent this tendency becomes. Some crystals showing the tendency described above are many of the mixed crystals of the III - V compound semiconductors such as Ga.sub.1.sub.-x Al.sub.x As (x representing a mixing proportion), GaAs.sub.1.sub.-x P.sub.x, Ga.sub.1.sub.-x Al.sub.x P, In.sub.1.sub.-x Al.sub.x As, and so on. In particular when a thick grown crystal layer of such substances is needed, variations in the composition of the layer, i.e. variations of the mixing ratio in the direction of growth provoke bending, cracking and so forth due to differences in lattice constants and variations in thermal expansion coefficients. When such a grown crystal layer is used for fabricating light emitting diodes, these variations often give rise to fluctuations in the wavelength of emitted light, fluctuations in the transmission of emitted light, and fluctuations in electrical properties as well. This naturally decreases the yield rate of fabrication of these diodes.
It is very advantageous from an industrial point of view to remove these difficulties by maintaining a predetermined mixing ratio or by keeping concentration of impurities constant over the whole thickness of the grown crystal layer.
In order to remove these difficulties, a method has been proposed, in which a temperature gradient is formed in a solution for crystal growth. The procedure for this method is hereinafter briefly described. A solution for crystal growth is heated at first to a temperature higher than that necessary for the beginning of crystal growth. Then the solution is cooled to the temperature necessary for the beginning of crystal growth. During this period of time small solid crystals form in the solution. The temperature is highest at the upper surface of the solution, and decreases gradually downward. A substrate crystal is placed at the bottom of the solution where the temperature is lowest. The small solid crystals are dissolved again in the solution due to the presence of the temperature gradient. The solute coming from the small solid crystals is transported by diffusion toward the substrate crystal and participate in an epitaxial growth on the substrate crystal.
Since the small solid crystals contain a large amount of the element having a great segregation coefficient, and since the decrease of the amount of this element in the solution, which takes place as the crystal growth on the substrate crystal proceeds, is compensated by dissolution of the small solid crystals, the concentration of the element in the solution is maintained at an almost constant level. One can thus obtain an epitaxial layer of homogeneous composition with respect to the mixing ratio as well as to the impurity concentration.
However this method in which a temperature gradient is formed in a solution for crystal growth has a great disadvantage in that it is difficult to obtain a thick epitaxial layer several millimeters thick, because it is otherwise desirable to keep the amount of small solid crystals in the solution small. When the amount of small solid crystals in the solution is too great, a part of them can be present in the neighborhood of the substrate crystal and they form recesses on, or are attached to, the surface of the substrate crystal. Since the amount of small solid crystals must increase with increasing thickness of the epitaxial layer to be grown, it is difficult to obtain a thick epitaxial layer having a flat surface by this method.